1. Field of the Invention
The present invention relates to a supervisory system for monitoring an optical amplification repeater unit in an optical communications network to which wavelength division multiplexing (WDM) technology is applied.
2. Description of the Related Art
Recently, an optical communications network operated using the wavelength division multiplexing technology (WDM) has been frequently taken up in many studies and developments. In the optical communications network, a large capacity data transmission can be realized using a wavelength-multiplexed optical signal.
On the other hand, a fault in an optical amplification repeater unit is a serious problem in the optical communications network. Therefore, an optical transmission system using the optical amplification repeater unit requires a unit for monitoring the state of the optical amplification repeater unit.
A state of the optical amplification repeater unit to be monitored refers to an optical input/output power, an exciting power, the temperature of an exciting laser diode (LD), a bias current of the exciting LD, etc.
A conventional optical transmission system has a point-to-point type transmission line in which a terminal station A is connected to another terminal station B one to one through a plurality of optical repeater amplifiers REP 1 through REP 4 as a point-to-point type transmission line as shown in FIG. 1. The system normally includes at least one optical wavelength multiplexing/demultiplexing unit; a plurality of terminal stations connected to at least one optical wavelength multiplexing/demultiplexing unit through an optical transmission line; and a plurality of optical repeater units positioned in the optical transmission line, wherein the second terminal station receives a supervisory signal (SV signal) transmitted from the first terminal station of the plurality of terminal stations to the optical repeater unit through the optical wavelength multiplexing/demultiplexing unit, and the received supervisory signal is transferred to the third terminal station through the optical wavelength multiplexing/demultiplexing unit. Otherwise, the system includes at lease one optical wavelength multiplexing/demultiplexing unit; a plurality of terminal stations connected to at least one optical wavelength multiplexing/demultiplexing unit through an optical transmission line; and a plurality of optical repeater units positioned in the optical transmission line, wherein the second terminal station receives a supervisory signal (SV signal) transmitted from the first terminal station of the plurality of terminal stations to the optical repeater unit through the optical wavelength multiplexing/demultiplexing unit; the received supervisory signal is transferred to the third terminal station through the optical wavelength multiplexing/demultiplexing unit.
In such a supervisory system in the above described optical transmission system, a supervisory signal (a command signal to an optical amplification repeater unit) is transmitted from the terminal station A to the optical amplification repeater unit REP 1. In such a conventional 1-channel communications (1-wave transmission), the optical amplification repeater unit REP 1 which receives the supervisory signal performs control corresponding to the received supervisory signal, and transmits a response signal containing the state information of the optical amplification repeater unit to the adjacent optical amplification repeater unit REP 2 for the supervisory signal to be sent to the terminal station B. Thus, the supervisory signals are sequentially transmitted.
A system for transmitting a supervisory signal and a response signal to an optical amplification repeater unit can be a superposing system for superposing the supervisory signal (command signal for controlling the optical amplification repeater unit) into a data signal (main signal) to be transmitted, or a wavelength division multiplexing system for wavelength-multiplexing the supervisory signal having a wavelength different from that of the main signal.
FIGS. 2A through 2C show an example of the operation of the optical amplification repeater unit in the superposing system. As shown in FIG. 2A, for example, the main signal M of 2.5 through 20 Gb/s is modulated by a supervisory signal SV of 1 through several tens MHz to generate a superposing signal, and transmitted to the optical fiber transmission line OP.
The optical amplification repeater unit branches an optical signal obtained by modulating the intensity of the main signal M using the supervisory signal SV, by the branch unit BR as shown in FIG. 2B. The branched optical signal is converted into an electrical signal by the optical receiving element PD such as a photo-diode, etc.
Only the supervisory signal SV is extracted through the filter FIL from among the electrical signals converted by an optical receiving element PD, and led to the control circuit CONT. The control circuit CONT controls an optical amplification repeater unit according to the extracted supervisory signal SV, and outputs a response signal SVxe2x80x2 having a predetermined frequency corresponding to the state of the optical amplification repeater unit. This response signal SVxe2x80x2 drives an exciting laser diode LD.
The gain of the optical fiber amplifier AMP is proportional to an exciting power. Therefore, the exciting power proportional to the response signal SVxe2x80x2 is output from the exciting laser diode LD, and the intensity of the optical signal input to the optical fiber amplifier AMP is modulated in proportion to the response signal SVxe2x80x2. Through this operation, the response signal SVxe2x80x2 from the optical amplification repeater unit can be superposed on the main signal M input as shown in FIG. 2C, and transmitted to the next optical amplification repeater unit from the optical amplification repeater.
On the other hand, various developments have recently been made to realize wavelength division multiplexing communications, and it is considered that the wavelength division multiplexing system (WDM system) will also be adopted in the future optical communications network. In WDM transmission, a plurality of channels can be transmitted through a single fiber. Therefore, a plurality of terminal stations can be connected by demultiplexing and multiplexing an optical signal.
In 1-wave transmission, an amplitude modulating method is used when an SV signal is transmitted. However, in the WDM transmission in which a plurality of terminal stations are connected, it becomes difficult to perform supervisory operation in the above described method on the following grounds.
(1) The modulation rate of the SV signal becomes smaller because of a lack of channels of a main signal produced by multiplexing/demultiplexing the main signal on which the SV signal is amplitude-modulated, thereby disabling the reception at a repeater unit.
(2) In multiple-repeater transmission, accumulated noise lights make the ratio of the power of a signal light to the entire optical power smaller. As a result, the modulation rate of the SV signal superposed on the signal light naturally becomes smaller.
(3) The propagation time of a signal light depends on the wavelength through the wavelength dispersion of a fiber. Therefore, the SV signal superposed on each signal light deviates in phase, thereby lowering the modulation rate.
(4) If the modulation rate in transmission from a terminal station is set to a high level in consideration of the modulation rate lowered in (1), (2), and (3) above, then the modulation of the SV signal affects the main signal.
FIGS. 3A and 3B show an example of an operation performed by the optical amplification repeater unit in the system in which the supervisory signal SV having a different wavelength from that of the main signal M is wavelength-multiplexed. FIG. 3A shows an optical spectrum and shows that the supervisory signal SV having a different wavelength from that of the main signal M is wavelength-multiplexed.
As shown in FIG. 3B, the wavelength-multiplexed light signal is input and branched by the branch unit BR, and the supervisory signal SV is extracted through the filter FIL. The extracted supervisory signal SV is led to a supervisory signal receiving circuit SVREC, opto-electrically converted, and demodulated.
Then, as with the example shown in FIGS. 2A through 2C, the optical amplification repeater unit is controlled according to the demodulated supervisory signal SV, and a response signal SVxe2x80x2 at a predetermined frequency corresponding to the state of the optical amplification repeater unit is output. The exciting laser diode LD is driven according to the response signal SVxe2x80x2.
The exciting power proportional to the response signal SVxe2x80x2 is output from the exciting laser diode LD, and intensity of the input optical signal of the optical fiber amplifier AMP is modulated in proportion to the response signal SVxe2x80x2. In this operation, the response signal SVxe2x80x2 can be transmitted from the optical amplification repeater unit together with the main signal M and the supervisory signal SV to the next optical amplification repeater unit.
FIG. 4A shows an example of the optical transmission system (WDM network system) which is a network system in the wavelength division multiplexing (WDM) technology. The transmission lines comprise at least a pair of optical fiber pair cable OPC as an up-line and a down-line for communications whose loss can be compensated by providing a plurality of optical amplification repeater units REP.
Each of the plurality of optical amplification repeater units REP is provided with optical amplifiers FF and FR. Furthermore, with the configuration shown in FIG. 4A, the optical wavelength multiplexing/demultiplexing unit OSEP is provided between the terminal stations A and D, and the terminal stations B and C for branching and inserting a signal are connected to the unit. A plurality of signal lights having different wavelengths are wavelength-multiplexed and transmitted as WDM signals from each of the terminal stations A, B, C, and D to a single optical fiber.
The WDM signal is transmitted from, for example, the terminal station A, distributed by the optical wavelength multiplexing/demultiplexing unit OSEP to its output path for each wavelength, and transmitted to the corresponding receiving terminal station B, C, or D.
FIG. 4B shows the concept of the path for an OTDR (Optical Time Domain Refrectometer).
The OTDR method is used to check the damage of an optical fiber and the state of the optical loss. In the OTDR method, an optical signal is transmitted from one optical fiber, and this optical signal propagated through the other optical fiber should be received. To attain this, an OTDR path is prepared to connect the up-line to the down-line at the optical amplifier provided in the repeater unit. This path is designed to simply connect the up-line to the down-line by branching an optical fiber. When the characteristic of the optical fiber is checked in the OTDR method, the output of a main signal and other signals are stopped to perform the check in the OTDR method. Then, the optical signal transmitted from the up-line is returned by each repeater unit to the source terminal station through the down-line. The optical fiber can be checked by detecting the returned optical signal.
FIG. 5 shows an example of a ring-shaped network. In this-example, as in FIG. 4A, the transmission lines comprise at least a pair of optical fiber pair cable OPC as an up-line and a down-line for communications whose loss can be compensated by providing a plurality of optical amplification repeater units REP. A WDM signal is distributed by the optical wavelength multiplexing/demultiplexing unit OSEP to its output path for each wavelength, and transmitted to a corresponding receiving terminal station.
The above described WDM networks shown in FIGS. 4A and 5 have the following characteristics. A plurality of signal lights (WDM signals) having different wavelengths are passed through a single fiber. The optical wavelength multiplexing/demultiplexing unit OSEP multiplexes/demultiplexes a main signal (WDM signal) in the transmission line. A plurality of terminal stations exist in a system, and a plurality of communications paths (optical paths) connects the stations.
However, when the system of transmitting/receiving a supervisory signal as described above by referring to FIGS. 2A through 2C, 3A, and 3B is applied to the system shown in FIGS. 4A and 5, the following problems arise.
A. Problem Arising when the Superposing System is Applied to the WDM Network
Even if a supervisory signal is superposed on a signal light having a specific wavelength, the signal light may not pass through all transmission lines. Therefore, a supervisory signal cannot be transmitted from a terminal station to all optical amplification repeater units in the network.
For example, in FIG. 6 shown correspondingly to FIG. 4A, only one of the up-line and the down-line is shown, but the transmission line of a supervisory signal can transmit the supervisory signal only to the optical amplification repeater unit REP provided between the terminal stations A and C even if the supervisory signal is superposed and then transmitted from the terminal station A by the wavelength W3.
Since the electric power of the signal light having a wavelength different from that of the signal light modulated according to the supervisory signal is strong, the ratio of the electric power of the supervisory signal in the electric power of all signal lights is reduced, and the receiving electric power becomes lower than in the single-wave transmission.
B. Problem Arising when the Wavelength Multiplexing/Demultiplexing System is Applied
When the communications path designed for transmitting a main signal is used as a communications path for exclusively transmitting a supervisory signal, a problem similar to the above described problem A arises. When a communications path is designed to connect a terminal station to all repeater units exclusively for supervisory control, signal lights having plural wavelengths are required.
For example, in the example shown in FIG. 7, as in FIG. 6, only one of the up-line and the down-line is shown. When a supervisory signal is transmitted from the terminal station A to all repeater units REP between the terminal stations A and B, A and C, and A and D, at least three wavelengths W1 through W3 are required. On the other hand, a method of branching a light of a supervisory signal can be adopted as shown in FIG. 8, but, in this case, the number of wavelengths should be equal to the number of terminal stations. Therefore, there is the problem that a transmission band cannot be effectively utilized.
Furthermore, only the light of a supervisory signal can be extracted using an optical filter in the supervisory signal receiving unit in the repeater unit to improve the electric power for receiving the supervisory signal in the repeater unit. However, when a plurality of wavelengths are modulated by the supervisory signal as described above, this method cannot be applied.
The problem similar to the above described problem A may arise when the wavelength division multiplexing system is used. Therefore, the present invention aims at providing a supervisory system in a wavelength division multiplexing network satisfying the following conditions.
That is, (1) The electric power for receiving a supervisory signal in the repeater unit is large, that is, the repeater unit can easily receive a supervisory signal; (2) There are no influences on a main signal; (3) An optional repeater unit can be monitored from a terminal station, and the signal transmission band can be effectively used.
A further object of the present invention is to provide an optical transmission system capable of transmitting a supervisory signal to all repeater units along a small number of channels in the WDM transmission, and checking an optical fiber in the OTDR method using a channel exclusive for a supervisory signal.
The first configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention that solves the above described problems includes at least one optical wavelength multiplexing/demultiplexing unit, a plurality of terminal stations connected to at least one optical wavelength multiplexing/demultiplexing unit through an optical transmission line, and a plurality of optical repeater units provided in the optical transmission line. With the configuration, a supervisory signal for the optical repeater unit transmitted from the first terminal station of the plurality of terminal stations is received through the optical wavelength multiplexing/demultiplexing unit by the second terminal station, and the received supervisory signal is transferred to the third terminal station through the optical wavelength multiplexing/demultiplexing unit.
The second configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the first configuration wherein the first terminal station transmits the supervisory signal to the second terminal station through the communications line assigned to main signal data. The second terminal station receives the supervisory signal, modulates the signal light having at least one wavelength according to the supervisory signal, multiplexes the modulated signal light having at least one wavelength with another signal light, and transfers it to the third terminal station through the communications line assigned to the main signal data.
Furthermore, the third configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the first configuration wherein the first terminal station modulates a signal light having at least one wavelength according to the supervisory signal, multiplexes a signal light having a plurality of wavelengths containing the modulated signal light, transmits the resultant signal light to the second terminal station. The second terminal station selects a wavelength from the received signal light, receives the supervisory signal, modulates a signal light having at least one wavelength according to the received supervisory signal, multiplexes the signal light having a plurality of wavelengths containing the modulated signal light, and transfers the signal light to the third terminal station.
The fourth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the first configuration wherein the first terminal station modulates a signal light having at least one wavelength according to the supervisory signal, transmits the signal light to the second terminal station. The second terminal station converts the wavelength of the received signal light into a different wavelength, and transfers it to the third terminal station.
The fifth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the third configuration wherein the second terminal station opto-electrically converts the signal light having at least one wavelength modulated according to the supervisory signal transmitted from the first terminal station to regenerate the supervisory signal, and modulates the signal light having a wavelength different from the at least one wavelength according to the regenerated supervisory signal.
The sixth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the third configuration wherein the second terminal station determines the necessity of transfer according to the supervisory signal, and stops the transfer of the supervisory signal when the transfer is not required.
The seventh configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the sixth configuration wherein the supervisory signal contains identification information which identifies the source terminal station and the destination optical repeater unit of a signal. The second terminal station reads the identification information about the supervisory signal, transfers the supervisory signal when the optical repeater unit exists in the transmission direction of the signal, and does not transfer the supervisory signal when the optical repeater unit does not exist.
The eighth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the first or third configurations wherein an optical path is formed in such a way that the supervisory signal transmitted from the first terminal station passes through the plurality of optical repeater units.
Furthermore, the ninth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention includes a first and a second terminal stations, a first optical transmission line for connecting the first terminal station to the second terminal station, at least one optical wavelength multiplexing/demultiplexing unit provided in the first optical transmission line, at least one of the third terminal stations connected to at least one optical wavelength multiplexing/demultiplexing unit through the second optical transmission line, and an optical repeater unit provided in the first and second optical transmission lines. In the wavelength division multiplexing communications network for establishing wavelength division multiplexing communications between the first and second terminal stations and at least one of the third terminals station, an optical path is formed as follows. That is, the first terminal station transmits an optical signal on which the supervisory signal for the optical repeater unit is superposed to one of the adjacent third terminal stations. The third terminal station transfers an optical signal on which the supervisory signal is superposed sequentially to the adjacent terminal stations, and transfers the optical signal on which the supervisory signal is superposed from the last third terminal station to the adjacent second terminal station. The second terminal station transmits the optical signal on which the supervisory signal is superposed to the first terminal station.
Furthermore, the tenth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the ninth configuration wherein an optical signal on which the supervisory signal is superposed passes through the first and second terminal stations and at least one of the third terminal stations, and has a fixed wavelength.
Furthermore, the eleventh configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention includes a first and a second terminal stations, first optical transmission up-line and down-line for connecting the first terminal station to the second terminal station, at least one optical wavelength multiplexing/demultiplexing unit provided in the first optical transmission up-line and down-line, at least one of the third terminals station connected to at least one optical wavelength multiplexing/demultiplexing unit through the second optical transmission up-line and down-line, and an optical repeater unit provided in the first and second optical transmission up-line and down-line. With this configuration, each of the first and second terminal stations and at least one of the third terminal stations transmits an optical signal on which the supervisory signal for the optical repeater unit is superposed to the adjacent terminal station through at least one of the optical wavelength multiplexing/demultiplexing units via the path which passes the corresponding optical transmission up-line and down-line.
Furthermore, the twelfth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the eleventh configuration wherein the second terminal station and at least one of the third terminal stations output with the wavelength different from the input wavelength of the input supervisory signal.
The thirteenth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention includes a first terminal station, an optical wavelength multiplexing/demultiplexing unit for wavelength-multiplexing/demultiplexing an optical signal transmitted from the first terminal station, a second and a third terminal stations for receiving the optical signal branched by the optical wavelength multiplexing/demultiplexing unit. With this configuration, the first optical path connects the first terminal station to the optical wavelength multiplexing/demultiplexing unit and is branched at the optical wavelength multiplexing/demultiplexing unit to the second and the third terminal stations The second optical path connects each of the second and the third terminal stations to the optical wavelength multiplexing/demultiplexing unit. Thus, the branches of the second path are combined at the optical wavelength multiplexing/demultiplexing unit and reach the first terminal station. Then, the first and the second optical paths pass the signal light modulated according to a supervisory signal.
Furthermore, the fourteenth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention includes a first and a second terminal stations, at least one of the first optical wavelength multiplexing/demultiplexing units provided in optical transmission up-line and down-line for connecting the first and the second terminal stations, a first path containing at least one of the third terminal stations connected to at least one of the first optical wavelength multiplexing/demultiplexing units through the optical transmission up-line and down-line, and a fourth and a fifth terminal stations, at least one of the second optical wavelength multiplexing/demultiplexing units provided in optical transmission up-line and down-line for connecting the fourth and the fifth terminal stations, a second path containing at least one of the sixth terminal stations connected to at least one second optical wavelength multiplexing/demultiplexing unit through the optical transmission up-line and down-line. In the optical wavelength division multiplexing communications network for establishing optical wavelength division multiplexing communications through the second path, an optical path is formed as follows. That is, the first terminal station transmits an optical signal on which a supervisory signal is superposed to one of the adjacent third terminal stations. The third terminal station transfers an optical signal on which the supervisory signal is superposed stage by stage to the adjacent terminal stations. Then, the optical signal on which the supervisory signal is superposed is transferred from the last third terminal station to the adjacent second terminal station. The second terminal station transmits the optical signal on which the supervisory signal is superposed to the fourth terminal station. The fourth terminal station transmits the optical signal on which the supervisory signal is superposed to one of the adjacent sixth terminal stations. The sixth terminal station transfers the optical signal on which the supervisory signal is superposed stage by stage to the adjacent terminal stations. The optical signal on which the supervisory signal is superposed is transferred from the last sixth terminal stations to the adjacent fifth terminal stations.
Furthermore, the fifteenth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the fourteenth configuration wherein an optical signal on which the supervisory signal is superposed passes through the terminal stations, and has a fixed wavelength.
The sixteenth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention includes a plurality of optical wavelength multiplexing/demultiplexing units connected in a ring through an optical transmission line, and a plurality of terminal stations respectively connected to the plurality of the optical wavelength multiplexing/demultiplexing unit. In the optical wavelength division multiplexing communications system, an optical path is formed in such a way that a light on which a supervisory signal is superposed passes through the plurality of adjacent terminal stations through the plurality of optical wavelength multiplexing/demultiplexing units.
Furthermore, the seventeenth configuration of the supervisory system in the wavelength division multiplexing communications network according to the present invention is similar to the sixteenth configuration wherein an optical signal on which the supervisory signal is superposed passes through the terminal stations, and has a fixed wavelength.
The eighteenth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention includes a plurality of terminal stations respectively connected to a plurality of optical wavelength multiplexing/demultiplexing units. The plurality of the optical wavelength multiplexing/demultiplexing units are connected in a ring shape through optical transmission up-line and down-line. In the optical wavelength division multiplexing communications system for establishing optical wavelength division multiplexing communications among terminal stations, an optical signal on which the supervisory signal is superposed is passed through a path connecting the adjacent terminal stations through the optical wavelength multiplexing/demultiplexing unit via an optical transmission up-line, and a path connecting the adjacent terminal stations through the optical wavelength multiplexing/demultiplexing unit via an optical transmission down-line.
The nineteenth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to the eighteenth configuration wherein the wavelength of the supervisory signal passing through the optical transmission up-line is different from the wavelength of the light passing through the optical transmission down-line.
The twentieth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention includes a plurality of optical wavelength multiplexing/demultiplexing units, a plurality of terminal stations respectively connected to a plurality of optical wavelength multiplexing/demultiplexing units. The plurality of the optical wavelength multiplexing/demultiplexing units are connected in a ring shape through optical transmission up-line and down-line. In the optical wavelength division multiplexing communications system for establishing optical wavelength division multiplexing communications among terminal stations, optical up-path and down-path connecting two adjacent terminal stations are formed. The up-path and the down-path are assigned to difference fibers. An optical main signal modulated according to the supervisory signal is passed through the optical path.
The twenty-first configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to the twentieth configuration wherein the wavelength of a signal in 2 N optical paths selected in transmitting a supervisory signal has a fixed wavelength.
The twenty-second configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to any of the first through twenty-first configurations wherein the light for a supervisory control is modulated according to a data signal, and the supervisory signal is superposed on the data signal.
The twenty-third configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to any of the first through twenty-first configurations wherein the light for a supervisory control is exclusively used for a supervisory signal.
The twenty-fourth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to any of the first through twenty-first configurations wherein a supervisory signal receiving unit in a repeater unit has a filter which passes only a signal light modulated according to a supervisory signal.
The twenty-fifth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to any of the first through third configuration of the present invention wherein an optical repeater unit includes a plurality of optical amplifiers, and each of the optical amplifiers which receives a supervisory signal transfers the supervisory signal to the other optical amplifiers.
The twenty-sixth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to the eleventh configuration includes at least one optical wavelength multiplexing/demultiplexing unit, a plurality of terminal stations connected to at least one optical wavelength multiplexing/demultiplexing unit through an optical transmission line, and a plurality of optical repeater units provided in the optical transmission line. A second terminal station receives a supervisory signal for the optical repeater unit transmitted from a first terminal station of the plurality of terminal stations through the optical wavelength multiplexing/demultiplexing unit, and transfers the received supervisory signal to a third terminal station through the optical wavelength multiplexing/demultiplexing unit. The wavelength of the supervisory signal passing through the terminal stations is fixed in all the terminal stations.
The twenty-seventh configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to the eighteenth configuration wherein the wavelength of the supervisory signal passing through the optical transmission up-line is the same as the wavelength of the supervisory signal passing through the optical transmission down-line.
The twenty-eighth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to any of the first through the twenty-seventh configurations wherein the wavelength of a signal modulated according to a supervisory signal can be switched depending on the position of a repeater unit to be monitored.
The twenty-ninth configuration of the supervisory system in the optical wavelength division multiplexing communications network according to the present invention is similar to the twenty-eighth configuration wherein the supervisory signal receiving unit of the repeater unit has a filter for passing only the light of a signal modulated according to the supervisory signal.
The supervisory system in the WDM transmission system according to the present invention is based on the configuration in which a hub station for issuing and terminating an SV signal, a branch station for looping back the SV signal, and a repeater unit for relaying an optical signal are connected through an optical transmission line. The SV signal is transmitted from the hub station along a channel exclusively used for the SV signal, and the SV signal is looped back by the branch station, thereby forming a path which passes the SV signal through all repeater units connected to the system.
The terminal stations used in the system according to the present invention includes at least a switch for switching an optical path and a repeater unit monitor device for generating an SV signal, and operates the switch to switch the transmission line when the SV signal is looped back and when the SV signal is transmitted or terminated. When the SV signal is transmitted, the SV signal, generated by the repeater unit monitor device is converted into an optical signal for transmission.
According to such a system or a terminal station of the present invention, it is not necessary to branch an SV signal and transmit it to each terminal station. That is, the optical path of the SV signal is changed to transmit the signal to a terminal station, and loop it back to pass through all the repeater units connected in the system. Accordingly, there is no problem of the deterioration of the SV signal from a branching operation, and the number of channels used in transmitting an SV signal is not so large as the number of the terminals.
Especially, according to the present invention, one channel can be used in transmitting an SV signal. Therefore, a number of channels are not exclusively required for the SV signal, or limit a transmission band. Therefore, a number of channels can be used in transmitting a main signal.
Furthermore, since a terminal station according to the present invention can either loop back an SV signal or transmit it, any terminal station can be a hub and branch station. Therefore, the SV signal can be transmitted through an appropriate transmission line formed depending on the position of a repeater unit to which the SV signal is transmitted.